First interpolate the 4 quads upper left corners, then sub-interpolate
each quad pixel. Do the perspective divide once per quad.
Saves some muls and reciprocates. But doesn't seem to make a
noticeable improvement.
It make the code simpler and more compact, so commiting anyway.
Eliminates all this identical yet slightly different code to decide how
shader inputs should be interpolated.
As bonus, don't interpolate the position twice when it is listed in the
TGSI shader inputs.
the llvmpipe tgsi translation is a lot more complete than what was in
gallivm so replacing the latter with the former. this is needed since
the draw llvm paths will use the same code. effectively the proven
llvmpipe code becomes gallivm.
This matches the convention used by the recursive rasterizer.
Also fixed assorted typos, comments, etc.
Now tri-z.c, gears.c, etc look basically right but there's still some
cracks in triangle rasterization.
Special attention is given to the interpolation of side by side quads.
Multiplications are made only for the first quad. Interpolation of
inputs for posterior quads are done exclusively with additions, and
perspective divide if necessary.
Vinson Lee
José Fonseca
Brian Paul
Zack Rusin
Keith Whitwell